Quadrupole assembly

A Quadrupole Assembly in Narrow Band-Pass Mode... 

For a beam of ions injected into the quadrupole assembly from an ion source, depending on mass, some (M,) pass along the central axis, but others (Mj, Mj) are deflected to the rods. 

End views of the quadrupole assembly (a) showing the theoretically desired cross-section and (b) illustrating the practical system. In (b), a positive potential, +(U + Vcoscot), is applied to two opposed rods (A) and a negative potential, -(U + Vcoscot), to the other two (B). The dotted lines indicate planes of zero electric field. The dimension (r) is typically about 5 mm with rod diameters of 12 mm. The x- and y-axes are indicated, with the z-axis being perpendicular to the plane of the paper. 

For a line OB (Figure 25.4) of smaller slope (smaller a/q or smaller U/V), an even greater range of m/z values will be transmitted through the quadrupole assembly. Conversely, for a line OC that passes through the apex (r) of the region of stability, no ions of any m/z value are transmitted. 

All mass spectrometers analyze ions for their mass-to-charge ratios (m/z values) by separating the individual m/z values and then recording the numbers (abundance) of ions at each m/z value to give a mass spectrum. Quadrupoles allow ions of different m/z values to pass sequentially e.g., ions at m/z 100, 101, 102 will pass one after the other through the quadrupole assembly so that first m/z 100 is passed, then 101, then 102 (or vice versa), and so on. Therefore, the ion collector (or detector) at the end of the quadrupole assembly needs only to cover one point or focus for a whole spectrum to be scanned over a period of time (Figure 28.1a). This type of point detector records ion arrivals in a time domain, not a spatial one. 

It is perhaps worth noting here that, if a quadrupole assembly is used in this all-RF mode, there is no significant mass separation as ions of different mass move through the guide. However, if a DC potential is applied to one pair of rods, the guiding potential changes to that shown in Equation 49.5, in which F is the applied DC potential. 

From Figure 49.2, it can be seen that the quadrupole assembly provides a potential well to contain the ions in their journey along the main quadrupole axis. The potential well of the quadinpole has not very steep sides and, compared with steep-sided hexapoles or higher -poles or ion tunnels, the quadrupole is not as efficient as the others in containing ions inside the rod assembly. 

Positive or negative ions (electrically charged species) from a source are injected along the central axis of the quadrupole assembly. 

If no DC (static) voltage is used, the remaining all-RF field guides all ions through the quadrupole assembly. There is no separation by m/z, and the quadrupole in this mode is often used as an ion/gas collision cell. 

As an ion enters the quadrupole assembly in z-direction, an attractive force is exerted on it by one of the rods with its charge actually opposite to the ionic charge. If the voltage applied to the rods is periodic, attraction and repulsion in both the X- and y-directions are alternating in time, because the sign of the electric force also changes periodically in time. If the applied voltage is composed of a DC... 

The resolution as adjusted by the U/V ratio cannot arbitrarily be increased, but is ultimately limited by the mechanical accuracy with which the rods are constructed and supported ( 10pm). [Ill] Above an m/z value characteristic of each quadrupole assembly, any further improvement of resolution can only be achieved at the cost of significantly reduced transmission. High-performance quadrupoles allowing for about 10-fold unit resolution have only recently been developed. [112]... 

Figure 1.4. Quadrupole assembly showing hyperbolic fields...

End views of the quadrupole assembly (a) showing the theoretically desired cross-section and (b) illustrating the practical system. In (b), a positive potential, +(U + Vcoscot), is applied to two opposed rods (A) and a negative potential, -(U +... 

Note that Equation 25.1 shows that the field (F) has no effect along the direction of the central (z) axis of the quadrupole assembly, so, to make ions move in this direction, they must first be accelerated through a small electric potential (typically 5 V) between the ion source and the assembly. Because of the oscillatory nature of the field (F Figure 25.3), an ion trajectory as it moves through the quadrupole assembly is also oscillatory. 

Passage through the quadrupole assembly is described as stable motion, while those trajectories that lead ions to strike the poles is called unstable motion. From mathematical solutions to the equations of motion for the ions, based on Equation 25.1, two factors (a and q Equation 25,2) emerge as being important in defining regions of stable ion trajectory. 

For small values of a and q, the shaded area in Figure 25.4 indicates an area of stable ion motion it shows all values for a and q for which ions can be transmitted through the quadrupole assembly. To gain some idea of the meaning of this shaded area, consider the straight line OA of slope... 

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